This invention relates to a method of manufacturing a hollow stabilizer, which decreases car rolling during turns. This invention relates, more particularly, to a method of manufacturing a hollow stabilizer made of heat-finished seamless steel pipe which is useful for a variety of small- and large-sized vehicles.
FIG. 14 is a perspective view which shows an embodiment of a stabilizer for a car and its use. The stabilizer 1 shown in FIG. 14 is U-shaped at a plan view and an intermediate part 2 thereof is active as a torsion bar spring, while both end portions 3, 3 of said intermediate part 2 are active as arms. Said intermediate part 2 is rotatably attached to a car body (not shown) through a rubber bush 4, and both of said ends 3, 3 are attached to suspension arms 7, 7 of wheels through link rods 5, 5, respectively. The stabilizer 1 increases riding comfort and driving stability by decreasing car rolling during turns.
The form of the stabilizer 1 can be many different types, e.g., a trapezoid, a reversal triangle or the like.
A hollow stabilizer has recently been developed in order to make car parts lighter and a hollow stabilizer made of seam-welded steel pipe has been used for vehicles having small weight loads, such as passenger cars and small-sized commercial vehicles. On the other hand, stabilizers made of non-hollow materials are used for vehicles having large weight loads, such as trucks, buses and the like, because the conventional hollow stabilizer cannot sustain the heavy loads of large vehicles such as trucks, buses and the like.
FIG. 15 is an explanatory chart which shows the relation in durability between a welding bead part and a part other than the welding bead when the seam-welded steel pipe is used for the hollow stabilizer.
As described above, the conventional hollow stabilizer is formed by a seam-welded steel pipe, said seam-welded steel pipe wound into a pipe and used by welding connection with electric resistant heating after extending band steel for mechanical construction and cold rolling. However, since a structure of the welding connecting portion forms a band structure of ferrite, even if quenched and tempered, the hardness of the welding bead portion is low and its portion decreases fatigue strength as shown in FIG. 15, whereby necessary durability cannot be obtained when it is used as a hollow stabilizer of a larger car. The results shown in FIG. 15 are a case tested at 45 Kgf/mm.sup.2 of stress.
FIG. 16 is an explanatory view which shows the relation between the depth of a linear crack in a spring steel and the fatigue life thereof, and FIG. 17 is an explanatory view which shows the influence a decarburization layer exerts on the fatigue strength of the spring steel.
Generally, when a spring steel is subjected to hot rolling, defects such as scab cracks or shock cracks occur as a result of inappropriate hot working, careless treatment of steel materials and the like, in addition to material defects such as linear cracks or the like. Further, when the steel is heated at a high temperature, its surface reacts with oxygen, carbon dioxide gas and vapor in an atmosphere to cause oxidation and decarburization.
As described above, when there exist such defects like linear cracks, scab cracks, shock cracks or the like, fatigue life is influenced even if the crack is shallow as shown in FIG. 16, illustrated with respect to a linear crack of a spring steel (tested by loading a torsion stress Tm.+-.Ta=48.+-.37 Kgf/mm.sup.2 on a drawing material having a 13.5 mm diameter).
Further, in said oxidation and decarburization, the decarburization generally advances faster than the oxidation, thereby forming a decarburization layer and an oxidation scale on the surface of the steel. In a spring steel having decarburization, even if a certain quenching is carried out, a sufficient hardness cannot be obtained and since there exists an area where the fatigue strength decreases suddenly as shown in FIG. 17, the depth of the decarburization must be as small as possible. An alloy component of the spring steel is shown in FIG. 17 wherein the spring steel (1) is C 0.53, Si 1.78, Mn 0.72 and spring steel (2) is C 0.58, Si 1.75, Mn 0.03.
Such phenomenon also occurs in seamless steel pipes of hot finishing. Accordingly, when the heat-finished seamless steel pipe is only used as stabilizer material, not only do cracks occur on the surface of the steel pipe, but also decarburization of 0.1-0.4 mm in length in the depth direction. Accordingly, the fatigue strength decreases sharply. Further, since the stabilizer uses the heat-finished seamless steel pipe by performing bending process, if the part having the surface defect described above is wound, the fatigue strength at the bending R portion decreases more violently.
Although there is a method of carburizing treatment and cold drawing work as ways to prevent decarburization, such methods are expensive (about 3 times the cost of heat treatment) and poor in utility.
FIG. 18 is a side view of a conventional pipe bender, FIG. 19 is a side view which shows it in operation, FIG. 20 is an enlarged sectional view of the stabilizer subjected to bending process by the pipe bender, and FIG. 21 is an explanatory view which shows the relation between a pipe coefficient .lambda. (bending degree) and a stress increasing rate proposed by Karman (relation between the pipe coefficient .lambda. and pipe elements shows the formula 1. The pipe coefficient .lambda. is constructed by a pipe thickness ratio t.sub.0 /d.sub.2, a bending ratio R.sub.0 /d.sub.2 and a central radius of bending R.sub.0). EQU .lambda.=4(t.sub.0 /d.sub.2 .multidot.R.sub.0 /d.sub.2).times.(1-t.sub.0 /d.sub.2).sup.-2.
The pipe bender 11 in FIG. 18 and FIG. 19 is provided with a rotational bending type 12, a holder type 13 arranged at the inner side of the bending center of the steel pipe to be worked and a pressure type 14 and a clamp 15 arranged at the outer side of a bending center of said steel pipe 10. Said pressure type 14 and clamp type 15 are arranged at a fixed interval, and provided so as to be movable around the rotational bending type 12.
Further, the material steel pipe 10 is inserted between a group of the rotational bending type 12 of the bender 11 and a group of the pressure type 14 and the clamp type 15 whereby the rotational bending type 12 is allowed to rotate and allowed to move the pressure type 14 and the clamp type 15 around the said rotational bending type 12. At this time, said pressure type 14 and clamp type 15 move pressing the bending portion R of said steel pipe 10 to the rotational bending type 12 thereby performing a bending process to said steel pipe 10 at a certain angle.
When the hollow stabilizer is manufactured by bending the steel pipe with pipe bender 11, the sectional form of the bending R portion is formed as a long and flat sectional surface in a vertical direction with respect to an axis which combines the inner side E and the outer side F of the bending center as shown in FIG. 20.
When the bending portion R of the hollow stabilizer is formed as said flat sectional surface, the stress increases according to the degree of bending as shown in FIG. 21 whereby the durability decreases according to the increase in stress. Accordingly, the hollow stabilizer wherein only the steel pipe is subjected to the bending process is unsuitable for a larger vehicle on which large loads are exerted.
The main object of this invention is to provide a method of manufacturing a hollow stabilizer by using a heat-finished seamless steel pipe as a material which is light in weight, able to withstand large loads, great in fatigue strength, superior in durability and able to lower costs.
Another object of this invention is to provide a method of manufacturing a hollow stabilizer having a stronger fatigue strength and durability.